Among the different possible ways to convert polymers into films, the blown film process with air-cooling is probably the most economical and also the most widely used. This is because films obtained by blowing have a tubular shape which makes them particularly advantageous in the production of bags for a wide variety of uses (e.g. bags for urban refuse, bags used in the storage of industrial materials, for frozen foods, carrier bags, etc.) as the tubular structure enables the number of welding joints required for formation of the bag to be reduced when compared with the use of flat films, with consequent simplification of the process. Moreover, the versatility of the blown-film technique makes it possible, simply by varying the air-insufflation parameters, to obtain tubular films of various sizes, therefore avoiding having to trim the films down to the appropriate size as is necessary in the technique of extrusion through a flat head.
Currently over 21 billion pounds of plastics are used in the U.S. each year for packaging. High density polyethylene (HDPE) blown films represent a substantial portion of this total. The blown film process is a diverse conversion system used for polyethylene. ASTM defines films as being of less than 0.254 mm (10 mils) in thickness; however, the blown film process may produce materials as thick as 0.5 mm (20 mils). It is important to produce HDPE films having high melt strength, good mechanical properties, and ease of processing that enable blown extrusion in structures with good bubble stability.
In order to increase the blown film bubble stability of bimodal polyethylene film material, the addition of peroxides in the extrusion system induces long chain branching (LCB) and improves the processing performance. Other free radical initiators such as oxygen may be used. The resulting color of the materials may be very yellow under some processing conditions. The processing window for yellowness is narrower when peroxides are used.
Several applications for HDPE include, but are not limited to, industrial bags, bags for frozen foods, carrier bags, heavy-duty shipping sacks, mailing envelopes, shrink films, among others. There is a constant need for materials having improved properties for particular applications.
It would be desirable if methods could be devised or discovered to provide polyethylene film or sheet materials having improved properties, particularly reduced color, such as reduced yellow color.